The following changes have been made to the VirtQueue API used by
the slave:
1. Add a vqId parameter to VirtQueue_create to make the index of
the vq explicit and not depending on the invocation order.
2. Add a len param to VirtQueue_addUsedBuf so users can control it.
3. Add a *len param to VirtQueue_getAvailBuf so users are notified
with the amount of received data

The MessageQCopy module which currently uses the VirtQueue API has
also been modified to adapt the new API